The present invention relates to an information recording medium with information recording thin films which is capable of recording information, which is obtained by subjecting an analog signal relating to an image or voice for example to frequency modulation, or digital information relating to data of an electronic computer, a facsimile signal, or a digital audio signal for example at a real time by utilizing a recording energy beam such as light, a laser beam, or an electron beam, and more particularly to a wavelength-multiple optical recording medium in which the position of wavelength of a light beam to be applied, such as an absorption spectrum or a photo-luminescence excitation spectrum of a recording medium, is changed to record the information.
Since the width of the wavelength in which the optical characteristics are changed is vary narrow, the wavelength-multiple recording can be performed by only changing the wavelength of the light beam emitted from the light source in the narrow range.
The description will hereinafter be given to the principle, which has been known from long ago, of the wavelength-multiple recording by the hole-burning. The hole-burning is caused in the system in which the photoabsorption centers such as chromophore molecules or color-centers (called the guest) are diffused into the medium having an irregular structure such as an amorphous material, or a crystalline material having a colorcenter (called the host). Since the structure of the host is irregular, the photoabsorption spectra of a large number of guests are superimposed to provide the spread of the spectra to define an absorption band. When the laser beam having the wavelength within that absorption band is applied to the system, the photoabsorption corresponding to that wavelength of the absorption band is decreased. The portion in which the photoabsorption is decreased is called the hole. At a very low temperature near 2K, the relative larger width of the absorption band is 40 nm and the width of each hole is 0.02 nm or so. Therefore, a multiple recording of a multiplex number of 1000 or so can be expected.
In recent years, efforts for increasing a recording/reading temperature of the hole-burning memory to make the memory to be readily used have been continued. In the year 1990, as described in an article Extended Abstracts (the 51st Autumn Meeting, 1990): THE JAPAN SOCIETY OF APPLIED PHYSICS No. 3, p. 1001, 26a-ZC8 (Sep. 26, 1990), the recording/reading of the hole-burning memory is enabled to be performed at a temperature of 77K (i.e., at a liquid nitrogen temperature). However, at a temperature above 100K, it becomes difficult to perform a reading operation.
In 1991, two reported in an article OPTICS LETTERS, Vol. 16, No. 6, pp. 420-422, 1991 by S. Arnold, C. T. Liu, W. B. Whitten and J. M. Ramsey that if the chromophore is diffused to the vicinity of the surface of the balls of 10 .mu.m or so diameter which are made of polystylene or latex and of which sizes vary slightly with the ball and the light beam is applied to the balls while changing the wavelength thereof step by step, the fluorescence having a strong intensity is observed from only the ball in which a certain relationship is established between the radius thereof a and the wavelength .lambda.. This is, as shown in FIG. 1, considered to result from an effect that the optical standing wave which has the strong electromagnetic field in the ball and in the vicinity of the surface thereof is present (i.e., the resonance is generated) thereby to enhance the optical absorption. In accordance with this principle, even if not at a very low temperature, the width of the photoabsorption spectra of one, or plural balls each having a certain radius becomes very narrow, so that the broad absorption band can also be formed by utilizing the distribution of the radii of a great number of balls. Moreover, if a light beam having the specific wavelength within that absorption band is applied, only the photoabsorption near that wavelength is decreased, the hole is formed or not formed in the same position on the recording medium in correspondence to "1" or "0" of the information signal to be recorded while changing the wavelength inch by inch, whereby the hole-burning memory can be realized at room temperature. In this connection, if the position on the recording medium to which the light beam is to be applied is also changed in sequence, it is possible to realize the recording of a very large capacity.
Moreover, although the wavelength-multiple recording is not described at all, it is reported in an article IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, Vol. AP-30, No. 2, pp. 168-172 (1982) by P. W. Barber, J. F. Owen and R. K. Chang that the resonance phenomenon as described above may also be generated in a circular cylinder or elliptic cylinder in the same manner. In this case, only the resonance in one solitary circular cylinder or elliptic cylinder is considered.